1. Field of the Invention
The present invention relates to a serial type inkjet printing apparatus. The invention particularly relates to an image processing method and image processing apparatus for reducing the bad effects of seam lines that appear at the boundary portions for each printing scan in a serial type printer.
2. Description of the Related Art
In a serial type inkjet printer, images are gradually printed by alternately repeating a main scan of moving while discharging ink, and a conveyance operation of conveying a printing medium in a direction crossing to the main scan. When there is fluctuation in conveyance amount during a plurality of conveyance operations while printing, seam lines such as white stripes or black stripes between the images (between bands) that are printed in each scan occur, which causes the image to become degraded. For example, when the conveyance amount of printing medium becomes greater than a specified value, white areas where no dots are printed become exposed and a white stripe occurs. On the other hand, when the conveyance amount of the printing medium is less than a specified value, more dots than needed are printed over each other in the boundary portion and a black stripe occurs.
It is known that such seam lines can be reduced a certain extent by applying multi-pass printing that completes an image of a unit area by performing multiple main scans. In multi-pass printing, image data that can be printed in one main scan are completed in multiple printing scans, then, when doing this, a conveyance operation having an amount that is shorter than the printing width of the printing head is performed between each printing scan. Therefore, an area of the printing medium that becomes a boundary portion during a certain main scan does not becomes a boundary portion in another main scan, so seam lines that are characteristic of the boundary portion are reduced. In this kind of multi-pass printing, the more multi passes there are, the less noticeable the seam lines become. However, on the other hand, the number of main scans necessary for completing an image also increases, so more time is required for printing. In other words, there is a tradeoff relationship between reducing the seam lines and throughput of the image output.
In regard to this problem, Japanese Patent No. 4,217,651, for example, discloses a printing method for making seam line less noticeable even when performing multi-pass printing using a small number of multi passes. More specifically, Japanese Patent No. 4,217,651 discloses a method for avoiding the appearance of black stripes and white stripes concerned by changing the conveyance amount to a large amount or a small amount according to the printing duty of the boundary portion that is obtained from the image data. Japanese Patent No. 4,217,651 also discloses a method of adjusting the printing duty in actual printing corresponding to the boundary portion by correcting the image data according to the printing duty value. With Japanese Patent No. 4,217,651, by setting the conveyance amount to a large amount in the case of a printing duty for which black stripes are a concern, and setting the conveyance amount to a small amount in the case of a printing duty for which white stripes are a concern, it is possible to output a uniform image with no noticeable black stripes or white stripes.
However, even though the method disclosed in Japanese Patent No. 4,217,651 is effective in reducing the state of seam lines predicted from the printing duty, this method is not able to avoid black stripes or white stripes that suddenly appear due to fluctuations in the conveyance operation. In addition, in the method disclosed in Japanese Patent No. 4,217,651, it is possible to adjust how noticeable seam lines will be within the controllable range of the conveyance amount; however, it is not able to handle small seam lines that occur due to error in the conveyance amount.